Rich combustion conditions, soot

Soot. Emitted smoke from clean (ash-free) fuels consists of unoxidized and aggregated particles of soot, sometimes referred to as carbon though it is actually a hydrocarbon. Typically, the particles are of submicrometer size and are initially formed by pyrolysis or partial oxidation of hydrocarbons in very rich but hot regions of hydrocarbon flames conditions that cause smoke will usually also tend to produce unbumed hydrocarbons with thek potential contribution to smog formation. Both maybe objectionable, though for different reasons, at concentrations equivalent to only 0.01—0.1% of the initial fuel. Although thek effect on combustion efficiency would be negligible at these levels, it is nevertheless important to reduce such emissions. 

A 2-value smaller than 1 means that there is an excess of fuel in the mixture. In this case the air/fuel mixture is called rich. If more air is in the mixture than needed for a complete fuel combustion (2 > 1) the term lean mixture is used. Ideally the combustion is complete at 2 = 1. Real fuel cannot be combusted without an increase in CO and soot at 2-values smaller than 1.05. Due to changing operation conditions, for example a soiled burner, wear of the nozzle or leaky flaps, change of gas quality or changes of temperature and air pressure in the ambient atmosphere, the air/fuel ratio and thus flue gas composition can change over time. In order to minimize the risk of intoxication (see also chapter 5333), explosion and pollution real (uncontrolled) fuel burners are adjusted to operate far beyond this limit in the excess (lean mixture) region. However, unfortunately effi-... 

The exact synthetic chemistry which produces PAH in a fuel-rich flame is not well known, even today. It is clear, however, that PAH can be produced from almost any fuel burned under oxygen deficient conditions. Since soot is also formed under these conditions, PAH are almost always found associated with soot. As an example of the PAH assemblage produced by combustion systems. Figure 1 shows gas chromatographic mass spectrometry (GCMS) data for PAH produced by the combustion of kerosene ( ). The structures of the major compounds are also given in Figure 1. We draw the reader s attention to a number of features of this PAH mix-... 

Carbon chemistry occurs most efficiently in circumstellar and diffuse interstellar clouds. The circumstellar envelopes of carbon-rich stars are the heart of the most complex carbon chemistry that is analogous to soot formation in candle flames or industrial smoke stacks (26). There is evidence that chemical pathways, similar to combustion processes on Earth, form benzene, polycyclic aromatic hydrocarbons (PAHs) and subsequently soot and complex aromatic networks under high temperature conditions in circumstellar regions (27,28). Molecular synthesis occurs in the circumstellar environment on timescales as short as several hundred years (29). Acetylene (C2H2) appears to be the... 

Several recent publications suggest that new reaction paths are needed to model aromatic oxidation and combustion [156, 154, 175, 176] Zhang et al. [156] published an elementary reaction model for high-temperature benzene combustion under fuel rich conditions - near sooting environment. They report that the flame speeds for benzene could not be matched by their model and suggested that some important reaction paths may be missing. Shandross et al. 

Whatever the process of external combustion, it has been shown that the amount of harmfid emission can be minimized by carefid control of the fuel mixtures and the burning conditions (e.g. the combustion temperature). Flames near the sooting limit (i.e. those from fuel-rich mixtures whose conditions just define the onset of the release elemental carbon aerosol) exhibit a large number of intermediate species, such as H2 and CO, rather than H2O and CO2. Furthermore, depending on the fuel, incomplete combustion will lead to the likely production of a variety of hydrocarbons or aromatic compounds. 

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